FHvn4d - since you are having trouble judging the correct height to start the flare, get your instructor to fly straight and level at this height at normal auto speed and have a good look at what the world looks like at that height and where, when looking out the front window, the horizon is relative to the windscreen bar or instrument coaming. Then get her to climb and descend through that height to see if you can recognise it purely from external references. Once familiar with the 'picture' at flare height, get her to fly an auto and you tell her when to flare - if you get it right it's your turn.
The only place to look, as others have highlighted, is out the front window so you can accurately control the attitude and the rate of change of that attitude, (you don't drive down the road staring at the hood ornament after all so look well ahead when you fly).
If you can start the flare consistently at the same height then you can try varying the flare to suit the conditions.